B cell antigen receptors (BCR) control important cell fate decisions of the B-lineage including differentiation, proliferation and B cell survival. Recently, it was demonstrated that beside chronic lymphocytic leukemia and diffuse large B cell lymphomas of the activated B cell-type also major subgroups of Burkitt lymphomas (BL) critically depend on BCR expression. As intensive chemotherapy and anti-CD20 antibodies are the current treatment strategies, pharmacological interference with critical BCR signaling pathways might open up the possibility for targeted therapies in BL. Such targeted therapeutic approaches are needed in particular for elderly patients and patients with endemic BL in developing countries. In order to (i) identify putative drug targets in BL and (ii) to elucidate BCR induced processes, we characterized BCR signaling in BL by quantitative phosphoproteomics and transcriptome sequencing. We identified about 10.000 phospho-sites in various BL cell-types, of which more than 1000 in almost 600 proteins were regulated in a BCR-dependent manner leading to dynamic transcription of more than 100 genes. The identified BCR signaling effectors belong to various functional groups, of which kinases, transcription factors and cytoskeleton regulators are most represented and bioinformatic analyses revealed that 83% of the identified BCR effectors were not described in the context of B cells or lymphoma before.

For instance, we identified a cell surface receptor that is highly expressed in BL and whose expression is regulated by the transcription factor c-Myc which is known to be frequently dysregulated in BL. ShRNA mediated knockdown of the cell surface receptor in BL cells impaired phosphorylation of spleen tyrosine kinase (Syk) and B-cell linker protein (BLNK) upon BCR stimulation, most probably due to dysregulation of the cytoskeleton. By using stable isotope labeling with amino acids in cell culture (SILAC) we investigated in a comprehensive manner, how the newly identified BCR co-receptor influences BCR signaling. Our study is a considerable complement to recent genetic studies that elucidated the mutational landscape in BL as it reveals mechanisms of signaling addiction to non-mutated BCR effector proteins that have to be validated in the future.

Citation Format: Carmen Doebele, Jasmin Corso, Anjali Cremer, Silvia Muench, Julia Beck, Christof Lenz, Hanibal Bohnenberger, Astrid Wachter, Tim Beissbarth, Ekkehard Schütz, Hubert Serve, Henning Urlaub, Thomas Oellerich. Elucidation of B cell receptor signaling in Burkitt's lymphoma reveals novel signaling nodes with potential therapeutic relevance. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 50. doi:10.1158/1538-7445.AM2015-50